Traveling wave amplifier tube having novel stop-band means to prevent backward wave oscillations



United States Patent 31,290,236 TRAVEMNG WAVE AMPLHFEER TUBE HAVENGNUVEL S'iGF-EBAND MEANS TO PREVENT BACKWARD WAVE GSiIlLLATlONS WilliamL. Borden, Nevada Qity, ali., assignor to Varian Associates, Palo Alto,(Dalifi, a corporation of (Ialitornia Filed Dec. 30, 196d, Ser. No.7?,893 ill Claims. (Cl. Sid-3.5)

The present invention relates in general to electron discharge devicesand more particularly to improvements in the slow-wave structure whichcarries the radio frequency wave in a traveling wave amplifier tube.

Traveling wave amplifier tubes of the type to which the presentinvention is applicable comprise an electron gun adapted to generate anarrow beam of electrons mounted at one end of an elongated tubularenvelope, abeam collector mounted at the opposite end of the envelope,and between the two and surrounding the path of the electron beam thereis positioned a slow-wave guide which, in the form to which the presentinvention particularly relates, comprises a long, slender helix.Terminals are provided at the input or electron gun end ofr supplying tothe helix the high frequency signal which is to be amplified by thetube, and terminals for connecting to a load are provided at the outputor collector end of the helix. The dimensions of the helix, includingdiameter and pitch, along which the radio frequency wave is propagatedare such that the longitudinal phase elocity of the wave issubstantially equal to the velocity of the electron beam which isdirected longitudinally of the helix.

A difficulty encountered in the operation of a traveling wave amplifiertube embodying a single helix as a slow-wave structure concerns theproduction of unwanted oscillations by way of the so-called backwardwave. These backward wave oscillations result from the interaction ofthe electron beam with a Fourier component of the wave in which theenergy is propagating in the opposite direction to the beam. ThisFourier component has a phase velocity in the direction of the beam andsubstantially equal to its velocity. Such oscillations are possible inany traveling wave tube embodying a single helix as a slow-Wavestructure and particularly if the beam almost fills the helix.

instability caused by oscillations of this kind may be prevented byproviding slight perturbations or non-uniformities in the characteristicimpedance of the wave conducting helix. One means of obtaining suchperturbations is to provide internal grooves on the casing whichencloses the slowwave guide. Procedures for forming these internalgrooves are relatively difiicult and costly, however. This is especiallytrue when it is desired to use a slow-wave guide casing made of metalfor reasons of production, ruggedness, etc.

It is, therefore, an object of this invention to provide improvedmethods and apparatus for producing backward wave attenuation for thepropagating structure of a traveling wave amplifier tube.

One feature of the present invention is the provision of a filter typestop band which is introduced in the single helix circuit of a travelingwave amplifier tube by a perturbation in impedance approximately everyonedzfifizdfi Patented Aug. ltd, 1965 half wavelength of the backwardwave oscillation frequency.

Another feature of the present invention is the provision ofnon-uniformities in the cross section of a slow- Wave helix in atraveling wave amplifier tube.

Still another feature of the present invention is the provision ofnon-uniformities in the cross section of a slow-wave helix in atraveling wave amplifier tube wherein the non-uniformities lie along apath having the form of a spiral whose pitch is such that the spiralwill intersect the main slow-Wave helix approximately once for everyhalf wavelength of the helix at the frequency which is desired as thecenter of a stop band.

These and other objects and features as well as the advantages arisingfrom the present invention will be apparent from the followingdescription of a preferred embodiment thereof as shown in the drawingswherein:

FIG. 1 is a sectional view of a traveling wave tube embodying thepresent invention,

FIG. 2 is an elevational view enlarged of a wire or tape suitable foruse in the helix of FIG. 1, and

FIG. 3 is a dispersion diagram for traveling wave amplifier tube.

Referring to the drawings there is shown one embodiment of the inventionin a traveling wave amplifier tube which includes a hollow cylindricalmetal casing 10 for the slow-wave helix or helical conductor H and thesapphire helix support rods 12. The novel helix 11 will be more fullydescribed below. The metal casing 16} is closed at one end by acollector 13 and associated cylindrical support 14 and closed at itsother end by an electron gun structure 15, including the cathode l6, andassociated cylindrical anode 17.

The collector 13 has a central evacuation bore 18 terminated by apinch-oil tube 19 molded into the insulator plug 2d. Enclosing a portionof the collector l3 and the insulator plug 2t} is the thermallyconductive cylinder 21 and associated cooling fins 37 which provide alarge area for the transfer of heat to a surrounding cooling medium.

A magnetic focusing coil 23 is enclosed by aluminum cylinders 24 and theassociated aluminum annular closures 25 which are supported by theradially extending arms 26 and the aluminum ring 27.

In operation, cathode and heater potential are supplied by the leads 23and 29 to the electron gun 15 which projects a beam of electrons alongthe axis of tne casing 10. The beam of electrons is accelerated andfocused by the anode 17 and focusing coil 23 before being collected bythe collector 13. Radio frequency energy to be amplified is coupled tothe slow-wave helix 11 by input terminal 31 and associated coaxiallead-in 32. The amplified radio frequency energy is taken from theslowwave helix 11 at output terminal 33 and associated coaxial lead-out34 after having exchanged energy with the electron beam all along theslow-wave helix.

A. portion of the novel slow-wave helix 11 in FIG. 1 is more clearlyshown in FIG. 2. Notches 39 are placed at uniform intervals along oneside of the tape helix 11 and the notches 46* are placed at uniformintervals along the other side of helix 1]. so as to form discrete areasof non-uniform cross section.

The areas of reduced cross section formed by the notches 39 and 4d arestaggered so that a notch 39 in one turn of the helix 11 will bedirectly opposite a notch 40 on a successive turn forming enlargedopenings 41 between successive turns of the helix 11. The spacingbetween the openings 41 is selected so as to provide perturbations inthe characteristic impedance of the helix 11 every one-half wavelengthof the backward wave oscillation frequency. Such a spacing will exist ifthe successive openings 41 lie along the path having the form of aspiral whose pitch is determined by where P is the pitch of the spiralformed by the opening 41;, P is the pitch of the slow-wave helix 11, fis the frequency at which the slow-wave propagates one-half wavelengthper turn of helix, and f is the desired stop band center frequency.

The non-uniformities in the characteristic impedance of the helix 11resulting from notches 39 and 40 cause a certain amount of reflection ofenergy back toward the source from the point at which irregularityoccurs. Where these reflections occur at uniform intervals of onehalfwavelength they become cumulative, the reflected waves reinforce, andthe repeating structure becomes a band elimination filter. The result isan attenuation of waves at or closely adjacent to the half wavelengthfrequency. At frequencies materially different from the center of thestop band, the reflections are not cumulative and the attenuation fallsoff very rapidly. With the structure here disclosed the number ofperturbations is very large so that even a small attenuation per sectionwill have a cumulative result large enough to produce an effective stopband. The width of the stop band produced is related to both width anddepth of the notches 39 and 40. Therefore, the width of stop banddesired will be one factor in determining the dimensions of the notches.Certain mechanical and physical limitations must also be considered,however. The depth of the notches 39 and 40 must not be so great as tounduly weaken the tape helix 11, for example. Also, as the width of thenotches 39 and 4% becomes greater than the width of the ridges 42, thenthe ridges 42 will replace the notches in forming the non-uniformportions along the helix. In this case, the areas of increased crosssection formed by the ridges 42 would accomplish the same result as wasformerly done by the notches 39 and 40.

PEG. 3 shows a diagram in which wave velocity (V/c) is plotted along thevertical axis and Ka is plotted along the horizontal axis where Ka is anormalized parameter proportional to frequency and numerically equal tothe number of wavelengths per turn. Line 43 represents the forward wavevelocity and line 44 represents the backward wave velocity for aparticular helix. A forward wave amplifier operating with the electronvelocity synchronous to the nondispersive forward wave velocity of ahelix will produce backward wave oscillation at the frequencyrepresented by the dotted line 45. This is normally a frequency at whichKa equals 0.5. A spiral path formed by the openings 41 which wouldproduce a stop band for this frequency would have an infinite pitch.Such a stop band could be easily produced by a once per turn notchcausing the openings 41 to form a straight line on the surface of thehelix 11. However, a traveling wave power tube is nearly always operatedat a different beam velocity than that represented by fiat portion ofthe forward wave curve 43, because of dispersion, space charge, andlarge signal effects. For example, a traveling wave tube operated at abeam velocity (Ve/c) represented by the dotted line 4-6 will producebackward wave oscillation at a frequency (F rep resented by the dottedline 47. A stop band at this frequency would require that the openings41 form a spiral having a pitch opposite to that of the helix 11 similarto that h wn in FI 1. On the other hand, operation of the tube at a beamvelocity less than that represented by the fiat portion of the forwardwave curve 43 would result in backward wave oscillations at a frequencyhaving Ka less than 0.5. This would require that the openings 41 beplaced so as to form a spiral having a pitch in the same direction asthe helix 11. In any case, the type of notch arrangement shown in FIG. 1may be designed to produce a desired stop band in a traveling waveamplifier tube without interfering with the interior structure of thetraveling wave tube or substantially affecting the forward wavecharacteristic impedance of its slowwave helix.

Many changes could be made in the above construction and many apparentlywidely different embodiments of this invention could be made withoutdeparting from the scope thereof. For example only, the tape helix shownin the drawings could be replaced by a wire helix, the notches formingnon-uniformities could be replaced by ridges, etc. It is, therefore,intended that all matter contained in the above description or shown inthe accompanying drawings shall be interpreted as illustrative and notin a limiting sense.

What is claimed is:

v 1. In a traveling wave amplifier device wherein energy is exchangedbetween an electron beam and the field of a radio frequency wave,apparatus for propagating the radio frequency wave comprising aslow-wave helical conductor, said slow-wave helical conductor havingdiscrete areas of non-uniform cross section at uniform intervals alongthe helical conductor to provide a stop band for backward waveoscillations, and wherein each of said discrete areas of non-uniformcross section lie along a path having the form of a spiral, and thepitch of said spiral is different than the pitch of said slow-wavehelical conductor.

' 2. A traveling wave amplifier device as defined in claim 1 wherein thepitch of said spiral is substantially equal to 7,,P /f f, where P is thepitch of said slow-wave helical conductor, f is the frequency at whichthe slow wave propagates one-half wavelength per turn of helicalconductor, and f is the center frequency of said stop band for backwardwave oscillations.

3. A traveling wave amplifier device as defined in claim 1 wherein saiddiscrete areas of non-uniform cross section are formed by areas ofreduced cross section of said conductor.

4. A traveling wave amplifier device as defined in claim 3, wherein thespacing between said discrete areas of reduced cross section issubstantially equal to one-half wavelength of the backward waveoscillation frequency.

5. A traveling wave amplifier device as defined in claim 1 wherein saiddiscrete areas of non-uniform cross section are formed by areas ofincreased cross section of said conductor.

6. A traveling wave amplifier device as defined in claim 5 whereinthespacing between said discrete areas of increased cross section issubstantially equal to one-half wavelength of the backward waveoscillation frequency.

7. In a traveling wave amplifier device wherein energy is exchangedbetween an electron beam and the field of a radio frequency wave,apparatus for propagating the radio frequency wave comprising aslow-wave helical conductor, said slow-wave helical conductor havingdiscrete areas of non-uniform cross section at uniform intervals alongthe helical conductor to provide a stop band for backward waveoscillations, the spacing be tween said discrete areas of non-uniformcross section being substantially equal to one-half wave length of thebackward wave oscillation frequency.

8. A traveling wave amplifier device as claimed in claim 7 wherein eachof said discrete areas of non-uniform cross section lie along a pathhaving the form of a spiral, and the pitch of said spiral is differentthan the pitch of said slow-wave helical conductor.

9. A traveling Wave amplifier device as claimed in claim 8 wherein thepitch of said spiral is substantially equal to f P /f f where P is thepitch of said slow- Wave helical conductor, f is the frequency at whichthe slow-wave propagates one-half Wavelength per turn of helicalconductor, and i is the center frequency of said stop band for backwardWave oscillations.

10. A traveling wave amplifier device as claimed in claim 7 wherein saiddiscrete areas of non-uniform cross section are formed by areas ofreduced cross section of 10 said conductor.

References Cited by the Examiner 3/41 Llewellyn 315 -3.5 X 1 6 Lines315-35 X Dodds 3153.6

Johnson et a1 315--3.6

Poulter 315-3.5

Dodds et al. 3l53.6

Warnecke et a1. 3153.5

Webber 315-35 Lagerstrom et a1 3153.6 Wilmarth 3153.6

GEORGE N. WESTBY, Primary Examiner.

RALPH G. NILSON, Examiner.

7. IN A TRAVELING WAVE AMPLIFIER DEVICE WHEREIN ENERGY IS EXCHANGEDBETWEEN AN ELECTRON BEAM AND THE FIELD OF A RADIO FREQUENCY WAVE,APPARATUS FOR PROPAGATING THE RADIO FREQUENCY WAVE COMPRISING ASLOW-WAVE HELICAL CONDUCTOR, SAID SLOW-WAVE HELICAL CONDUCTOR HAVINGDISCRETE AREAS OF NON-UNIFORM CROSS SECTION AT UNIFORM INTERVALS ALONGTHE HELICAL CONDUCTOR TO PROVIDE A STOP BAND FOR BACKWARD WAVEOSCILLATIONS, THE SPACING BETWEEN SAID DISCRETE AREAS OF NON-UNIFORMCROSS SECTION BEING SUBSTANTIALLY EQUAL TO ONE-HALF WAVE LENGTH OF THEBACKWARD WAVE OSCILLATION FREQUENCY.